1. Field of the Disclosure
The present application relates to an organic electroluminescent display device.
2. Description of the Related Art
Recently, flat display device have been actively researched in order to enhance display quality and enlarge the screen size. Among the flat display device, organic electroluminescent display devices are self-illuminating display devices. In order to display images, the organic electroluminescent display devices display emit light by forming excitons images through the recombination of carriers such as an electron and an electric hole.
The organic electroluminescent display device can be used as a pixel of a graphic display system, an image display device of a television system, a surface light source, or others, because of having features such as a wide viewing angle, high speed response, a high contrast ratio and so on. Moreover, the organic electroluminescent display device with light weight, thin thickness and superior color vision is considered to be a next generation display device.
Also, the organic electroluminescent display device is classified into a passive matrix type and an active matrix type. An organic electroluminescent display device of the active matrix type includes thin film transistors, but an organic electroluminescent display device of the passive matrix type does not include any thin film transistor.
Such an organic electroluminescent display device includes organic light emission elements each formed in pixel regions. Each of the light emission elements includes an organic light emission layer which is interposed between an anode and a cathode and formed from an organic light emission material.
The organic light emission layer of the organic light emission element includes a plurality of functional layers, such as a hole injection layer, a hole transport layer, a light emission layer, an electron transport layer, an electron injection layer or others. The combination or arrangement of functional layers can enhance the light emission efficiency of the organic light emission element.
The above-mentioned organic light emission layer is ordinarily formed in the pixel region of the organic electroluminescent display device using a vacuum deposition process which allows an organic light emission material to be deposited on a substrate.
In the vacuum deposition process, an organic light emission material used to form the organic light emission layer is placed into a deposition source with an outlet. The deposition source is heated within a vacuum chamber and enables the organic light emission material to be vaporized and discharged through the outlet. As such, the vaporized organic light emission material can be discharged from the deposition source and deposited on the substrate.
If the organic electroluminescent display device including a plurality of organic light emission layers with desired patterns is fabricated, the deposition process is performed using a shadow mask having a plurality of openings. More specifically, the shadow mask is positioned closely to the substrate. Then, the vaporized organic light emission material is deposited on the substrate through the shadow mask. Therefore, the organic light emission layer having a plurality of desired patterns which are separated from one another can be formed on the substrate.
FIG. 1 is a view illustrating an ordinary process of forming an organic light emission layer of the organic electroluminescent display device. As shown in FIG. 1, the organic electroluminescent display device includes red, green and blue organic light emission layers R, G and B which are formed in an active area. The red, green and blue organic light emission layers are formed on the substrate 10 by heating a boat into which the organic light emission material is filled.
More specifically, when a substrate 10 in which organic light emission layers will be formed is loaded into a chamber, a shadow mask 30 placed on a mask frame 15 is aligned with the substrate 10. Then, boats 20 holding red, green and blue organic light emission materials are sequentially heated. As such, the red, green and blue organic light emission layers R, G and B are sequentially formed on the substrate 10. As the boats 20 are sequentially heated, shadow masks each opposite to red, green and blue sub-pixels are replaced alternately with one another in order to form the red, green and blue organic light emission layers R, G and B in red, green and blue sub-pixel regions.
However, when the shadow masks 30 are used in the formation of the organic light emission layers, the organic light emission materials cannot be uniformly deposited on the substrate 10. To address this matter, the distance between the shadow mask 30 and the boat 20 can increase. In this case, the organic light emission layer can be polluted by other materials and moved apart from a desired formation position due to the contact of the shadow mask 30 with the substrate 10.
Moreover, a large sized organic electroluminescent display device forces the size of the shadow mask to become larger. The enlarged shadow mask can be bent with its central portion in the center. Due to this, it is difficult to accurately form the organic light emission layer.